swap cksum

#include <stdint.h>
#include <stdio.h>
#include <asm/byteorder.h>
#include <netinet/ip.h>

#define ipv4_addr(o1, o2, o3, o4) __constant_htonl( \
    (o1) << 24 | \
    (o2) << 16 | \
    (o3) <<  8 | \
    (o4))

static struct iphdr ip = {
    .version = 4,
    .ihl = 5,
    .id = __constant_htons(0xcda3),
    .frag_off = 0x40,
    .ttl = 64,
    .protocol = IPPROTO_UDP,
};

static uint16_t ip_checksum(uint16_t *buf, size_t len)
{
    unsigned long sum = 0;
    int i;

    for (i = 0; i < len; i++) {
        sum += htons(buf[i]);
        if (sum & 0x80000000)
            sum = (sum & 0xFFFF) + (sum >> 16);
    }

    while (sum >> 16)
        sum = (sum & 0xFFFF) + (sum >> 16);

    return htons(~sum);
}

static void calc_csum_print(struct iphdr *ip)
{
    ip->check = 0;
    ip->check = ip_checksum((uint16_t *)ip, ip->ihl * 2);
    printf("chk: %hx\n", ip->check);
}

int main(void)
{
    ip.saddr = ipv4_addr(192, 168, 0, 1);
    ip.daddr = ipv4_addr(10, 0, 20, 1);
    calc_csum_print(&ip);

    ip.saddr = ipv4_addr(0, 1, 192, 168);
    calc_csum_print(&ip);

    ip.daddr = ipv4_addr(20, 1, 10, 0);
    calc_csum_print(&ip);

    ip.saddr = ipv4_addr(10, 0, 20, 1);
    ip.daddr = ipv4_addr(192, 168, 0, 1);
    calc_csum_print(&ip);

    return 0;
}